Could be abcess... Since I don't know how old you are, it is hard to say. If you are in your twenties then it could be your wisdom teeth erupting if you still have them. If you don't still have your wisdom teeth, or you are older, then it could be a gum infection, with a small abcess. People can also get tumors on their gums, especially if you smoke. You should see a dentist.
It could be anything. If this bump doesn't go away by itself after several weeks of observation and protecting it from trauma, you should see your doctor to assess this bump. Eventually, you may need to see an ENT if it doesn't go away by itself.